Rare plant species, establishment of exclosures and recommendations for a monitoring program in the Washington-Slagbaai Park (Bonaire)
The aim of the present project–commissioned by the management of the Washington-Slagbaai Park (WSP)- was to set-up a more strategic management of the natural elements of this protected area.
The results are presented in three sections: I. Rare plant species; II. Exclosures and III. Monitoring. The survey of the rare plants resulted in the discovery of three plant species not previously known for Bonaire: Capparis linearis (a tree), Adiantum capillus-veneris (a fern) and a yet undetermined Thelypteris sp.( (a fern). Occurrences were also documented for rare species already known to occur in the WSP. The survey also led to the rediscovery of Capparis tenuisiliqua for the WSPand Bonaire. Brassavola nodosa is reported for the first time for the WSP.
The search for rare plants took place on a number of the higher hills in the WSP. The ‘Matadó di Pasku’ was the hill with the most rare plant species (15). Most of the higher hills in the WSP have a number of denuded areas or areas with scanty vegetation. This is the result of vegetation and soil degradation caused mainly by goat grazing. The negative impact of these animals is also reflected in the almost complete absence of seedlings or other stages of rejuvenation of the rare plants and the exclusive occurrence of the orchid Brassavola nodosa on rock boulders at inaccessible heights for goats.
Isolation of vegetation results in reduction of its dispersal capabilities. As a way to counteract this it is recommended to fence off small areas around the rare tree species identified during the survey. A number of indigenous plant species have been identified that should receive priority in a monitoring and recovery plan: Capparis tenuisiliqua, C. linearis, Schoepfia schreberi, Maytenus tetragona, Eugenia procera, Myrcia curassavica, Psidium sartorianum and Celtis iguanaea. A monitoring protocol is presented to aid in the collection of the data. It has also been recommended to start a propagation program for the orchid species B. nodosa and Schomburgkia humboldtii. The latter was reported in a botanical study in 1994 and must occur in small numbers but could not be relocated. While Sideroxylon obovatum, Guapira fragans and G. pacurero were considered rare in a botanical study done in the WSP in 1994 we did not consider these species rare because they occur quite frequently on the hills visited and are also found in other areas of the island. The results of the present study indicate that it would be valuable to have a botanical survey done of the higher hills of the WSP that could not be visited in the present study.
Three exclosures (9 x 9 m) and three adjacent control plots of the same size (accessible to goats, donkeys and pigs) have been established in the dominant Casearia-Prosopis vegetation type of the D3 landscape type of the WSP. Data collection took place in nine sub-plots of 3 x 3 for both the exclosures and adjacent control sites. The control site was the area of which the vegetation most resembled the vegetation inside the exclosure.
TL7 was the second landscape type in which exclosures and control sites were established. TL7 was chosen above the second most frequent landscape type (D2) of the WSP because its exclosures will provide insight into the development of limestone vegetations and effects of introduced animals (mammals) on them. Limestone vegetations are characterized by dominance of (semi-)evergreen plant species. The sample plots surveyed are characterized by a low number of plant species and high incidence of goat dung pellets.
It is recommended to repeat the data sampling of the exclosures and control sites in two years order to gain insight in the short-term changes in the different vegetation types. Longer-term changes will be seen over a number of years.
To gain a broader insight in natural vegetation development (which will also help to understand development in animal communities) it is necessary to establish several 50 x 50 m permanent plots in (at least) the two most dominant vegegation types in the WSP: Casearia-Prosopis type (D3) and Croton-Haematoxylon type (D2). Because the Slagbaai area will be the first part of the WSP where the goats will be removed, these plots should be placed in this part of the WSP. A data sheet to aid in the data collection (developed by ITC) is presented. A total understanding of the natural development of the vegetation of the whole WSP will be possible through the making of a detailed vegetation map (scale 1:10,000-1:25,000) and repeating it after 20 -30 yrs.
Only one bat species (Glossophaga elongata) is found in the two caves and a well in the WSP. A monthly monitoring program of especially the cave on Seru Grandi is needed in order to determine its significance for G. elongata and possibly other bat species occurring on Bonaire. The easily accessible small cave at Slagbaai should be fenced off to prevent unnecessary disturbance of the bats roosting there by visitors of the WSP.
The monitoring top priorities for bird species are: Buteo albicaudatus, Amazona barbadensis, Sterna hirundo, S. antillarum and Phoenicopterus ruber. Monitoring methods are presented for these species.
No data is available on the impact of exotic plant species in the WSP, but it must be low because invasives were not recorded in any of the sample plots from the park used for the production of the vegetation map of the island and Cryptostegia grandiflora was only seen on the Juwa. In order to be able to control the impact of these species (Cryptostegia grandiflora, Leucaena leucocephala, Azadirachta indica and Jasminum fluminense) they should be monitored. Deleterious exotic animals causing damage in the park at present include goats, donkeys, pigs, cats and dogs. These should be controlled and monitored also. It is therefore recommended to make it a duty of the WSP rangers to report on plant or animal exotic species seen during their patrols and fieldwork. A design for a simple monitoring protocol to record and monitor exotic species is presented. A quick assessment by a biologist of the level of presence of exotic plant species in the WSP will help form the basis of such a program for the exotic plant species and determine the necessary level of interventions by park management. At present there is a trapping program for goats in place using a number of fenced off areas in Slagbaai.
Among the group of remaining animal species three species have been indicated as indicator species for monitoring: Poecilia vandepolii (euryhaline fish), Gecarcinus ruricola (land crab) and Phoebis agarithe (butterfly).
Six habitat types have been distinguished in the WSP: Cave systems, salina habitat, freshwater habitat, vegetation types of Washikemba formation, vegetation types of limestone formations and beach landscape. Their characteristics, occurrences in the WSP, importance for the survival of native fauna and flora species and management and monitoring options are given for these habitat types.